Force transmission devices of disc brakes, particularly air actuated disc brakes, are mounted within a caliper of the brake and transmit the actuation force from the actuator to friction elements of the brake via tappets or pistons. Typically, force transmission devices also incorporate an adjuster mechanism to compensate for wear of the friction elements in use.
Various arrangements of force transmission device have been employed by air disc brake (ADB) manufacturers. Knorr-Bremse utilise SB and SN series brakes utilise externally threaded spindles that are screwed into a cross-member that is actuated by an operating shaft. The spindles are hollow and have a non-circular inner profile that interfaces with an adjuster mechanism to screw out the spindles to account for wear of the brake pads and maintain a desired running clearance between the brake pads and rotor. The outer threads are problematic for effective sealing of the interior of the brake caliper. An example of this arrangement can be seen in e.g., EP0648952.
The present applicant's ELSA2 series brakes utilise two-part pistons/tappets. The outer piston is smooth and has a trilobular profile to prevent its rotation within a complementary trilobular bush within a fixed guide plate. The inner tappet is threaded with respect to the outer piston such that rotation of the inner tappet causes the rotationally fixed outer piston to extend. By contrast with the Knorr-Bremse arrangement, the operating shaft of the ELSA2 brake transmits the actuating force directly to the inboard end face of the inner tappets.
Whilst the ELSA2 brake has been found to be highly reliable and robust in service the applicant has recognised that it is desirable to reduce the cost associated with machining the trilobular surfaces, the cost of providing seals to seal against the trilobular surfaces, and the cost of providing an inner tappet that is both a relatively complex shape and must be suitably treated to withstand the forces imposed during actuation. In addition the piston housing is presently formed from an aluminium casting having relatively costly trilobular machinings. The casting also acts as a cover plate for the rotor side of the housing which requires additional treatment to avoid corrosion issues.
The present invention seeks to overcome or at least mitigate the problems of the prior art.